Process for continuously melting and solidifying refractory materials



J. L.. DELRlEux 3,484,511 PROCESS FOR CONTINUOUSLY MELTING AND SOLIDIFYING Dec. 16, 1969 REFRACTORY MATERIALS 2 Sheets-Sheet 1 Filed 001'.. 18, 1965 L Nam m 7////.//// ////////////f///// //N/f///V//////// //////4 ,r

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y@ w1 lo@ 'n United States Patent Or 3,484,511 PROCESS FOR CONTINUOUSLY MELTING AND SOLlDIFYIN G REFRACTORY MATERIALS `lean Louis Delrieux, Le Pontet, Vaucluse, France, assignor to LElectro-Refractaire, Paris, France, a company of France Filed Oct. 18, 1965, Ser. No. 497,350 `Claims priority, application France, Dec. 29, 1964, 231 Int. Cl. ll28b 1/14, 1/54 U.S. Cl. 264-57 4 Claims ABSTRACT F THE DISCLOSURE Method of continuously melting and solidifying refractories, which comprises continuously passing a bed of granular or pulverulent refractory compound first through a melting zone then through a solidifying zone, wherein the speed of movement of said bed, the heat input in the melting zone and the heat loss immediately after the melting zone are adjusted to produce a solidification front which is substantially fixed in space, and wherein an unmelted matrix of fritted refractory particles is left at the sides and below the molten zone.

Since their introduction, electrically cast refractories have been intermittently produced by the meltin-g of the refractory material or mixtures of refractory material in an electric furnace, followed by the casting of the molten refractory product into moulds in Which it solidifies into shaped pieces. Each mould takes a volume of liquid which is inadequate for the volume of the final solid product, which contracts as it solidifies, the contraction causing shrinkage cavities in the resultant solid product.

To obviate this disadvantage during casting, one or more casting heads filled with liquid are disposed above the mould. It has also been suggested to keep the material in the feeding head molten, for instance by means of an auxiliary arc, to fill the shrinkage cavities as they occur during the solidification of the refractory block.

However, it has never proved possible to completely avoid shrinkage cavities in electrically cast refractories. To obtain absolutely solid blocks, those portions thereof which contain residual shrinkage cavities have to be sawn off, and this means a Very high loss of cast and moulded material.

In the metallurgical industry, which is faced with a similar problem, it has been suggested to use continuous casting methods in which molten metal is regularly and continuously cast from a casting ladle or a melting furnace via a flow regulator into a vigorously cooled open bottomed receptacle from which there emerges continuously and at a suitably controlled speed a billet whose still liquid centre is finally solidified by a secondary cooling. The billet is subsequently cut up by chisels or torches.

It would be difficult to use a method of this kind for refractory products, since the constituents of which they are composed require very high temperatures-in the order of 2,000 C. and moreto melt them and keep them in the liquid state. They very quickly acquire a pasty constituency as soon as they have left the heating source.

The method according to the invention, which solves the problem of continuously melting and solidifying refractory materials, is characterised in that the initially granular pulverulent oxide or mixture of oxides is continuously passed first through a melting Zone which comprises a heating source and in which the oxide is heated by high-frequency electric induction, and then through a solidifying zone immediately downstream of the melting zone, the speed of the movement of the material at the outlet from the melting Zone being such that taking into 3,484,511 Patented Dec. 16, 1969 Tice account the heat in the input to the melting zone and the heat loss immediately after leaving the melting zone, a solidification front is produced which is fixed in space and which is in permanent contact with the liquid material in the melting zone.

The method according to the invention obviates all transference of a volume of liquid from one receptacle to another and enables the shrinkage cavities to be continuously filled with the liquid material in contact with them, the result being a solid refractory billet which can then be cut up into blocks, for instance by diamond cutting wheels. The method according to the invention also obviates moulding, which is a very expensive operation in the case of electrically cast refractories having high melting points, i.e., 2,000 C. and above.

The method according to the invention can be used for the continuous melting and solidilication of refractory components of all kinds. Examples are refractory compounds based on refractory oxides, such as alumina, zirconium oxide, magnesia, thoria, etc., or refractory compounds such as carbides, nitrides, borides, etc. In the case of the latter, however, since they are generally sensitive to the effect of the atmosphere at elevated temperatures, the molten material in the melting zone has to be protected from any contact with the atmosphere by surrounding the melting zone with an inert gas.

In one advantageous embodiment of the method according to the invention, the heating source, whose power is selected in relation to the energy required to melt the material and the required production per unit time, is applied to the bed of charge material, the shape and crosssection of which charge material are such that the molten zone has a cross-section corresponding to that of the refractory product to be obtained, the surface of the molten zone being at a lower level than the top of the bed of charge material, and the molten zone is to be found only in the top central portion of the cross-section of the bed of material (the effective cross-section), thus leaving at the sides and below the molten zone an unmelted layer formed by particles of raw material sintered by the heat from the melting zone. The layer protects the walls of the receptacles which receive the material for melting so that they can be made of steel without any particular precautions being necessary.

The eective cross-section of the bed of material and its speed of movement are determined by the required production per unit of time, and the heat input is calculated in relation to the required production per unit of time and the nature of the material to be melted. The three parameters, namely the heat input, the total crosssection of the bed of charge material, and its speed of movement in the melting Zone, are inter-related.

In the solidifying zone downstream of the melting zone, the cross-section of the resulting refractory product lacquires a shape and size conditioned on the one hand by the lower cross-section of the bed of charge material, and on the other by the control of cooling at its top part, since the molten material has to be quickly provided with a measure of resistance to possible deformations caused by atmospheric pressure and the load of heatinsulation introduced in the following zone. Otherwise, not only would geometrical deformation occur, but there is also a risk that some liquid zones may become isolated and shrinkage cavities formed therein. The degree and duration of cooling of the top of the material in course of solidification required to provide the refractory with suitable mechanical properties before it enters the annealing zone which follows, is determined experimentally.

The invention also covers the devices used in the performance of the method according to the invention and the end products obtained.

For instance, a suitable apparatus may comprise a very long trough which receives the material for melting and moves continuously into the zone of action of a heating source for melting, and then into a solidifying zone, preferably followed by an annealing zone.

An exemplary non-limitative embodiment of the invention iwill now be described with reference to the accompanying drawings, the details to be gathered from both the drawings and the accompanying description.

FIGURE 1 is a diagrammatic vertical section through the axis of an appartus for the performance of the method according to the invention,

FIGURES 2 and 3 are partial views, illustrating two different phases of operation.

FIGURE 4 is a cross-section, taken along the line IV-IV in FIGURE l, through the zone in which the bed of material for melting is shaped.

FIGURE 5 is a cross-section, taken along the line V-V in FIGURE 1, through the melting zone,

FIGURE 6 is a cross-section, taken along the line Vi- VI in FIGURE 1, through the solidifying zone, and

FIGURE 7 is a cross-section, taken along the line VII-VII in FIGURE 1, through the annealing Zone.

zontal longitudinal members 4 located at a certain distance I' above the ground on uprights 5. The rollers 3 can rotate freely on their shafts and the trough 1 can be passed slowly over the rollers 3, from the left to the right as illustrated in FIGURE 1, by any means, for instance by a motor-driven chain and belt.

The pulverulent or grannular refractory compound or mixture of refractory compounds 6 for melting is poured into the trough 1, for instance by a spout 7. A fixed vertical gauge 8 prepares the bed of material for melting by giving it a concave, for instance, an inverted W-crosssection, as illustrated in FIGURE 4, as the batch of material for melting passes below the gauge 8 as the trough 1 moves. A heating source 9 for melting takes the form of two electrodes 9a of the kind normally used in electric arc furnaces. In the embodiment illustrated the electrodes 9a are disposed one after the other in the vertical plane of symmetry of the system. For melting, the ends of the electrodes 9a engage in a triangular hollow made in the top of the material by the gauge 8 (FIGURE 4). The arc l is first struck between the tips of the two electrodes 9a and is then broken up into two arcs, one between each of the electrodes 9a and the material for melting, when the conductivity of the material, which is low when cold, has become adequate as a result of the rise in the temperature of the material. The material is thus melted.

FIGURES 2 and 5 show the start of melting. The `arc has been struck as stated hereinbefore and the material has begun to melt, forming beneath the electrodes a pocket of molten material which is separated from the trough walls by unmelted material whose grains are fritted and which protects the trough walls. The trough 1, whose right-hand end only is charged, has remained stationary during the initiation of melting and now begins to move continuously towards the right. Its speed of movement is controlled in relation to the power of the heating source and the cross-section of the bed of material, so that meltting is transmitted to the solid material arriving from the left below the fixed electrodes, without the molten material occupying the whole cross-section of the trough 1. The object of this step is also to protect the trough walls and it of course requires a suitable adaptation of the dimensions of the cross-section. As the trough 1 moves, the molten material 4passes at the right out of the Zone of action of the electrodes, and the top of the molten ma- 4 terial begins to solidify, as shown in FIGURE 2. A cavity 10 is therefore formed which is filled with still liquid material and whose V-shape gradually becomes accentuated as a result of the increase in thickness of the solidied crust, until equilibrium is reached between the amount of heat contributed to the molten material and the loss of heat at its top to atmosphere. From then onwards conditions are established and the solidification front formed by wall 11 of the cavity 1d is fixed in space (FIGURES 3 and 6). Along the solidification front there is permanent contact between the solid and liquid material, and no cavities are formed in the material, as is the case when refractory material is cast into moulds.

A feeding funnel 12 disposed some distance downstream of the electrodes 9a is opened when the solidified product begins to pass below and pours on to the solidified material a certain thickness 13 of the granular heat-insulating product (which can be refractory compound or mixture of refractory compounds itself), with the result that during the remainder of its travel to the right, as illustrated in the drawings, the solidied slab 14 of refractory material slowly completes its cooling.

The distance at which the feeding funnel 12 is disposed downstream of the electrodes 9a controls the loss of heat from the material leaving the melting zone and must be suitably adjusted so that the solidified top crust above the cavity 11 is rigid enough not to collapse under the heat-insulating load or the action of atmospheric pressure.

As the material passes from left to right, as illustrated in the drawing, it moves through four successive zones; Zone A-the zone where the material for melting is charged and the bed of material for melting is prepared; Zone B the melting zone; Zone C-the solidifying Zone; and Zone D-the annealing or slow cooling zone.

The method is continuous and can be formed on an endless chain of supports for the trough. When a trough reaches the end zone where annealing is complete, the layer of unmelted material is removed from the refractory slab and the slab is cut up, for instance sawn up, into blocks of the required shape. The trough is then returned to the head of tr e continuous line to be relled with material for melting without this operation interrupts the continuity of the production of the refractory slab.

Clearly, the power of the heating source, the speed of movement of the bed of material for melting, and the loss of heat in the solidifying zone are interdependent and vary in relation to certain parameters, such as the nature of the refractory material to be treated, the shape and surface of the cross-section of the required product, the thickness of the unmelted matrix, and the distance of the electrodes from the heating source. It is therefore impossible to provide general directions on this subject, and each case must be taken on its own merits. However, an engineer in the art will be able to select suitable operational conditions for each particular case.

The following are some exemplary non-limitative numerical values which have been used in the laboratory tests:

EXAMPLE l Continuous melting -and solidication of a batch com prising about 45-60% MgO and 15-30% Cr2O3, the remainder substantially comprising A1203, Fe2O3, SiO2 and CaO.

The power of the heating source used was of the order of 40G-450 kva.

Speed of advance: S0 cm. per hour.

Cross-section of the molten zone: semi-circular, with a surface -area of the order of 4 dm?.

Thickness of the sintered layer: about 4 cm.

Thickness of heat-insulation layer (unmelted and unagglomerated batch): about l5 cm.

EXAMPLE 2 Continuous melting and solidication of a batch having a high magnesia content (8S-98% MgO):

The power of the heating source used was of the order of 450 kva.

Speed of advance: 80 cm. per hour.

Cross-section of the molten zone: semi-circular, with a surface -area of the order of 2 dm?.

Thickness of the sintcred layer: about 4 cm.

Thickness of heat-insulation layer (unmelted and unagglomerated batch): about l5 cm.

The embodiment of the invention described hereinbefore is merely exemplary and could not be modified, more particularly by the substitution of technically equivalent means, without exceeding the scope of the invention.

Any other kind of conveyor may be substituted for the trough 1.

The embodiment of the invention which has been described hereinbefore enables a substantially semi-circular or square refractory product to be obtained. Of course, the apparatus can be adapted to produce a fairly wide sheet, i.e., to give some transverse continuity so as not to have too many parallel production lines, To this end, a certain number of electrodes can be aligned transversely in the melting Zone to multiply the arcs.

The heating source can be other than an electric arc. For instance, plasma torches or any other available source can be used.

The cross-section of the bar-shaped or sheet-shaped resulting refractory product can also be modiled to ensure the minimum loss of material when the refractory product is cut up. To this end, for instance, the field of temperature gradients can be adjusted, or the refractory material can be passed through -a water-cooled gauge on leaving the melting zone.

The annealing device hereinabove described and comprising the feeding funnel 12 which covers the solidified material with a certain thickness of heat-insulating material, can be replaced by a tunnel kiln kept at a suitable temperature through which the solidified product continuously moves.

The installation can be completed by the provision of devices for continuously sawing up the resultant product or cutting it up by plasma torch. Such devices can either produce the blocks in final form, or rough out masses from which the final blocks will subsequently be cut.

I claim:

1. A method of continuously melting, solidifyng, and cooling refractory material to form fused cast refractories wherein said material consisting essentially of loose particles of one or a mixture of oxides, carbides, nitrides, and borides of aluminum, zirconium, magnesium, chromium, iron, silicon, calcium, and thorium are deposited into a trough like receptacle and said material is moved together through a melting zone where a portion of the cross-section of said material is melted from the top and center by high temperature producing means leaving at the sides and below the molten zone a sufficient unmelted and sintered region of said material to protect said receptacle from said molten material, said receptacle of molten and unmelted material is continuously moved out of the melting zone and immediately into a solidifying zone; the speed of movement of said receptacle of said material, the heat input to the melting zone, the rate of heat loss in the solidifying zone, and the cross-section of the bed of said material are interdependently controlled to produce a molten zone of the desired cross-sectional area, correspending to that of the resultant refractory product plus allowance for cooling shrinkage, and a solidiication front xed in space and in constant contact with said molten material, and the horizontal column of solidified refractory and said protective layer in said receptacle are continuously moved together through a cooling zone where the temperature of the `fused cast refractory is reduced at conventional controlled rates, said melting, solidifying and cooling steps being conducted in an atmosphere inert to said material.

2. The method as set forth in claim 1 wherein the top of the at least partly solidified column of fused cast retfractory is covered with a granular heat insulating material after the top of said column has acquired a solidification thickness sufficient to support its own weight and the weight of the insulating material without deformation.

3. A method as set forth in claim 1, wherein the top of the material is shaped to a concave form in cross-section before it enters the melting zone.

4. A method as set forth in claim 1, wherein the top of the material is shaped to an inverted W in cross-Sectio before it enters the melting zone.

References Cited UNITED STATES PATENTS 1,220,839 3/1917 Gray 264-27 1,430,724 lO/ 1922 DAdrian 264-27 3,286,309 11/1966 Brondyke et al. 164-82 3,332,740 7/1967 Alper et al 264-332 3,338,988 8/1967 Accary et al. 264--332 3,352,350 11/1967 Dore et al 164-82 FOREIGN PATENTS 1,471,001 3/1966 France. 37/3,898 8/1962 Japan.

IULIUS FROME, Primary Examiner JOHN H. MILLER, Assistant Examiner U.S. Cl. X.R. 

